Cream together the butter and sugar in a large bowl until light and fluffy, about 5 minutes. Add eggs one at a time, beating well after each addition then stir in the milk and grapefruit juice. Mix in the flour, cinnamon, nutmeg, baking powder, and salt. Pour resulting batter into a 9x5 inch loaf pan.
Bake in preheated oven until a toothpick or small knife inserted in the crown comes out clean, about 45 minutes.
While the cake is baking, prepare the icing. Place the sugar in a bowl and gradually stir in the water. Add the grapefruit juice and salt; pour over warm cake. It should form a glaze as it cools.